The present invention relates in general to the field of detection, and more particularly, to a structure and method for protecting integrated circuit sensors from the environment of intended use.
Without limiting the scope of the invention, its background is described in connection with the protection of integrated circuit fingerprint sensors from the environment during regular use, as an example.
Heretofore, in this field, the detection of fingerprint patterns has been useful for the identification of specific individuals based on the observation that each individual person has a unique fingerprint. Fingerprints, therefore, can be used not only to positively identify individuals, but to exclude individuals whose fingerprint profile does not match a pre-existing set of patterns.
Fingerprint sensing has evolved from optical and mechanical sensing technology that acquires a fingerprint image. Generally, the mechanical and optical sensors obtained a fingerprint image using a scanner or a camera, processed the acquired information into an analog or digital signal that could be analyzed, and provided an output based on the acquired signal. Unfortunately, the lighting and contrast conditions available at the time the image was acquired affected the analysis and consequently the output from the sensor.
Another class of fingerprint sensors are capacitive sensors, such as that disclosed in U.S. Pat. No. 4,353,056 issued to Tsikos. The Tsikos patent demonstrates the use of a sensor that incorporates a sensing member that has a sensing surface for receiving a fingerprint. The sensing surface has a means for sensing the ridges and valleys of the skin of the finger under observation. A sensing member containing a multitude of capacitors that sense the patterns of the fingerprint when the finger is pressed against the sensing surface. The information obtained is transformed into an electric signal. The capacitors are insulated from the environment of use by a flexible membrane that conforms itself to the contour of the fingerprint. Unfortunately, the repeated cycles of flexing and compression of the flexible membrane can lead to device failure and the need to replace the membrane.
U.S. Pat. No. 4,353,056 issued to Ruell, et al., discloses a fingerprint sensor that provides an electric output signal in response to the topographic relief of the fingerprint. The sensor incorporates a contact body that is formed at least in part by a light transparent elastic material. The elastic contact material can be attached to a flat sensor plate that has a light receiving surface. The sensor also incorporates a light source and a photodetector to measure the valleys and ridges of the fingerprint. The elastic nature of the contact body, of the above described sensor causes cycles of compression and flexing that lead to the deterioration of the contact point between the sensor and the finger.
It has been found, however, that the present methods and structures for protecting fingerprint sensors from the environment of intended use fails to address the distinct environmental exposures to which the sensors are exposed. For example, under ideal conditions the user would gently place the finger on the plate without an excess of pressure or shock. Unfortunately, it is the case that the sensor surface will be exposed to a wide variety of pressures, but that objects other than fingers might come in contact with the sensor surface.
Another problem with current sensors is the need to protect the sensor from electrostatic discharges, e.g., static electricity, caused by the user and the sensor being at different voltage potentials. A user can be exposed to environmental conditions that cause a great increase in potential in comparison to objects that are at a different potential or grounded. When the user presses the sensor at a great voltage disparity, the sudden discharge may cause operational failure of the sensor, both temporary and permanent. The current flowing from the discharge may be small. However, damage to the data flowing from the sensor or to the sensor itself can still occur. While damage to the data or the sensor should be avoided, the sensitivity of the sensor should be maintained at close to optimal levels.
Yet another significant problem of current structures for the protection of fingerprint sensors are the contaminants, such as oils and proteins that are found on the surface of fingers. To remove these contaminants it will be the case that organic and inorganic solvents and detergents will be needed to clean the sensor surface.
Another area of concern is hygiene. Fingers, as well as the environment, tend to contain a number of microbes that need to be removed from the sensor along with finger contaminants. To remove these materials and reduce the chance of contagion between users, antibacterial, antifungal and decontaminating agents are used to clean the sensors. These decontaminating agents can often include harsh abrasives, enzymes, organic and inorganic solvents and detergents. Furthermore, the sensors are exposed to oxidating environments, UV rays, and the like during normal use.
What is needed is a structure and method to protect fingerprint sensors from electrostatic discharges, while at the same time maintaining the sensors, ability to withstand mechanical stress. The structure must not only permit continued functioning of the sensor during normal use, but be able to withstand, among others, the extreme conditions of humidity, electricity, heat, light, etc., to which the sensor may be exposed. The sensor structure should also be resistant to chemical detergents and solvents, but be compatible with the underlying components of the sensor.
In one embodiment, the present invention is directed to an integrated circuit passivation comprising, an integrated circuit, over which an insulating layer is disposed. The insulating layer helps to electrically isolate the integrated circuit from subsequent layers and the environment. Next, a discharge layer is formed that is electrically or semi-electrically conductive. The discharge layer dissipates electrical discharges that may be caused when a user touches the sensor. A mechanical protection layer may also be disposed over the discharge layer to provide hermetic and mechanical protection for the underlying circuit. The discharge layer may comprise a silicon-based layer that is partially doped to increase conductivity. In one embodiment the discharge layer is an SiCx layer, where x is less than 1.
One preferred use of a circuit for use with the present invention is a fingerprint sensor, which can be, e.g., a capacitively coupled fingerprint sensor. In one embodiment, the mechanical protection layer and the discharge layer are the same, and can comprise a mixture of silicon carbide and a SiCx layer, where x is less than 1. In yet another embodiment of the invention, the mechanical protection layer and the discharge layer are formed at the same time and have a chemical formula of SiCx, wherein the stoichiometry of the carbide component is varied throughout the deposition of the discharge layer in order to optimize the conductivity and maximize the hardness of the mechanical protection layer.
The present inventors recognize that the sensitivity of an integrated circuit sensor can be degraded by adding a highly conductive layer, such as a metal layer. Likewise, it is herein recognized that the discharge layer or sensor surface should be resistant to mechanical stress caused by environmental conditions and use, e.g., scratches. Therefore, the inventors electrically isolate the functional components of the sensor, for example, capacitors, by disposing a first insulating layer. In one embodiment, the sensor is protected from electrostatic discharges by a passivation that can integrally contain one or more semi-electrically conductive layers, with at least one layer being a discharge layer. Preferably, the discharge layer is an SiCx layer, where x is varied to maximize hardness and optimize conductivity. In another preferred embodiment the discharge layer is a silicon-based layer that is doped to optimally bleed electrical discharges, while at the same time maintaining sensor device sensitivity. The mechanical protection layer and the discharge layer should be compatible and, in one embodiment, can be disposed on the insulating layer concurrently. Concurrent deposition of the mechanical protection layer and the discharge layer can be accomplished by, for example, beginning the preposition with SiC and thereafter decreasing the amount of carbon to make a SiCx layer, where x is less than 1.
In another embodiment, the insulating layer of the integrated circuit passivation can be a silicon oxide layer. Alternatively, the first insulating layer can be silicon nitride. The integrated circuit passivation may further comprise a second insulating layer disposed between the first insulating layer and the mechanical protection layer. The second insulating layer can be silicon oxide or silicon nitride. In yet another embodiment the insulating layer is silicon oxide, and the second insulating layer is silicon nitride, with the discharge layer whose hardness is greater than that of silicon nitride being partially conductive. A mechanical protection layer can also be disposed above or below the discharge layer.
Yet another embodiment of the present invention is a method of fabricating a fingerprint sensor passivation comprising the steps of, obtaining an integrated circuit, depositing an insulating layer on the integrated circuit and depositing over the first insulating layer a discharge layer that is semi-electrically conductive. The method may further comprising the step of depositing a second insulating layer between the first insulating layer and the discharge layer, and may also include depositing a mechanical protection layer on or below the discharge layer. The mechanical protection layer and the discharge layer should be compatible and, in one embodiment, can be disposed on the insulating layer concurrently. Concurrent deposition of the mechanical protection layer and the discharge layer may be conducted, and can be accomplished by, for example, beginning the preposition with SiC and thereafter decreasing the amount of carbon to make a SiCx layer, where x is less than 1.